Reconciling the Systemic Kicks of Observed Millisecond Pulsars, Spider Pulsars, and Low-mass X-ray Binaries

TL;DR

This paper investigates the apparent discrepancy in systemic kicks among millisecond pulsars, spider pulsars, and low-mass X-ray binaries, proposing a distance-dependent bias correction that reconciles their evolutionary relationship.

Contribution

It models and corrects for kinematic biases due to distance, showing that observed systemic kicks align with binary evolution predictions, resolving previous tensions.

Findings

Corrected systemic kicks are consistent across MSPs, LMXBs, and spider pulsars.

Distance bias significantly affects observed systemic kick distributions.

All corrected kicks match binary population synthesis models.

Abstract

Millisecond pulsars (MSPs) have been proposed as evolutionary products of low-mass X-ray binaries (LMXBs) through a stage in which they are spider pulsars (i.e., redbacks and black widows). However, recent work has found that the systemic kicks of observed MSPs are significantly lower than the kicks of LMXBs and spiders, which appears to be in tension with this evolutionary model. We argue that this tension can be relieved, at least to some degree, by considering the fact that the observed MSPs are located at relatively short distances, whereas spider pulsars are located at greater distances and LMXBs are situated even further away. We model the distance-dependent kinematic bias for dynamically old objects, which favors observing objects that have received low kicks at short distances and correct the observed systemic kicks for this bias. We find that this kinematic bias can be big enough to close the gap between the MSP and LMXB kicks, although the spider pulsars appear to come from a slightly different systemic kick distribution, but this difference is not necessarily physical. All corrected systemic kick distributions are consistent with predictions from binary population synthesis for progenitor systems with a post-supernova orbital period of $P_{\text{orb}}\leq10\,$d and a companion mass of $M_{c}\leq1\,M_{\odot}$, where the natal kicks are calibrated to the velocities of young isolated pulsars. We conclude that the difference in observed systemic kicks is not necessarily in tension with a common origin for MSPs, spider pulsars, and LMXBs.